Anatomy - The key structures mediating penile erection

Anatomy

The key structures mediating penile erection are the paired corpora cavernosa
or 'erectile bodies' (Figure 1). These cylindricalstructures form the bulk
of the penis and fill with arterial blood under pressure at the time of
erection. Fused distally for three-quarters of their length, they separate
proximally to fuse with each ischial tuberosity of the pelvis. Each corpus
cavernosumhas a thick fibrous sheath, the so-called tunica albuginea, which
surrounds the erectile tissue, made up of multiple lacunarspaces that are
inter-connected and lined by vascular endothelium (Figure 2). The trabeculae
constitute the walls of thesespaces, and comprise smooth muscle and a
fibroelastic framework of collagen in almost equal quantities.

The corpus spongiosum surrounds the urethra, which traverses the length of
the penis within this structure, lying in theventral groove formed by the
paired corpora cavernosa in the pendulous portion. At its proximal portion, it
expands to formthe bulb, which curves upwards through the urogenital
diaphragm to reach the apex of the prostate gland. Distally, the
corpusspongiosum expands to form the glans penis (Figure 3). The spongiosum
is composed of sinusoidal spaces of largerdimensions than those of the
corpora cavernosa and with less smooth muscle. The tunica albuginea surrounding
thespongiosum is flimsy compared with that of the corpora, but the
spongiosum is nevertheless capable of an erectile response.

The corpus spongiosum in the bulbar region is surrounded by the
bulbospongiosus muscles (Figure 4). These have twoimportant functions: to
facilitate ejaculation by their rhythmic contractions, and to empty the bulbar
urethra after voiding,thereby preventing postmicturition dribble.

The skin overlying the penis is exceptionally mobile and expandable to
accommodate the considerable increase in girth andlength that occurs during
erection. This lack of adherence makes it relatively susceptible to edema. In
its distal portion, thepenile skin extends forward to form the prepuce
before folding backwards and attaching to the corona of the glans
penis(Figure 5).

The pendulous portion of the penis is supported and stabilized by the
suspensory ligament (Figure 6). Division of thisstructure makes the penis
appear longer in its flaccid state, but this does not enhance the proportions of
the organ when erect.

ARTERIAL BLOOD SUPPLY

The blood supply to each corpus cavernosum is derived mainly from the
internal iliac artery, a branch of the atheroma-pronecommon iliac artery. In
the pelvis, the internal pudendal artery passes beneath the sacrospinous
ligament and over thesacrotuberous ligament, and gives off the perineal
artery in Alcock's canal, where it runs under the superficial
transverseperineal muscle and the symphysis pubis (Figure 7). After giving
off the perineal artery, it becomes the common penile artery(Figure 8). This
vessel pierces the pelvic floor adjacent to the inferior ramus of the ischium
near the bulb of the urethra andgives off the bulbar, urethral, dorsal and
cavernosal branches before reaching the corpus cavernosum to form one element
ofthe paired dorsal arteries (Figure 8).

The cavernosal artery on each side pierces the tunica albuginea at the hilum
of the penis. It then runs distally in the centerof each corpus while giving
off numerous helicine branches. These corkscrewshaped muscular vessels open
directly into thelacunar spaces (Figure 9). The tonic contraction of the
smooth muscle walls (Figure 10) normally allows only small amountsof blood
into the lacunar spaces, thereby maintaining penile flaccidity. Relaxation of
the muscular walls of these vesselsinitiates the hemodynamic changes that
result in penile erection.

VENOUS DRAINAGE

Blood leaves the penis via three venous systems: superficial, intermediate,
and deep. The superficial system allows blood frommultiple superficial veins
to drain into the superficial dorsal vein, which itself drains into the left
external branch of theinternal saphenous vein. The intermediate venous
system lies beneath Buck's fascia and comprises the deep dorsal vein andthe
multiple circumflex veins. This system drains blood from the glans, corpus
spongiosum and the distal two-thirds of thecorpora. The deep dorsal vein
runs in the groove dorsally between the corpora cavernosa. It enters the pelvis
beneath thesuspensory ligament, which suspends the corpora from the
undersurface of the pubic arch and drains into the dorsal venous